An investigation of a heterogeneous aminohydroxylation catalyst.

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Abstract

Os-Zn-Al hydrotalcite-like compounds (HTlc’s) were synthesised by the co-precipitation method
and characterised using different techniques (powder XRD, ICP-OES, FT-IR spectroscopy,
BET-surface area measurements, SEM and SEM-EDS, cryo-TEM, 27Al SS-NMR and
TGA-DSC). The hydrotalcite-like catalyst was used to heterogenise the aminohydroxylation
reaction. Among the three solvents investigated (toluene, MeCN/water (1:1 v/v) and t-
BuOH/water (1:1 v/v)) in the aminohydroxylation reaction, toluene showed the slowest reaction
rate, MeCN/water (1:1 v/v) and t-BuOH/water (1:1 v/v) demonstrated fast reaction rates
comparable to each other. The reaction temperature was only significant when toluene was used
as the solvent system (reaction time (100% depletion of starting material) and temperature are
inversley proportional). The catalyst HTlc structure demonstrated a significant effect in terms of
the reaction time and isolated yield of the -amino alcohols. Under the same testing conditions a
heat treated catalyst (non-HTlc) showed a shorter reaction time, a reduction in the isolated yield
of -amino alcohols with a rise in diol formation. All the different classes of olefins (aliphatic,
aromatic, and functionalised) that were tested, gave 99.99% depletion of starting material.
However, due to the same purification difficulties encountered in the homogeneous amino
hydroxylation (AA) reaction, the isolated yield of -amino alcohols achieved here, ranged from
13 to 35 %, with the highest yield (35%) obtained when methylcinnamate was used as the olefin.
Characterisation of the spent catalyst showed that HTlc structure is maintained, but crystallinity
was lost (the material becomes polycrystalline) after the reaction. The leaching test showed that
4.5% and 5.5% of Os leached from the catalyst to the reaction solution when MeCN/water (1:1
v/v) and t-BuOH /water (1:1 v/v) were used as the solvent system, respectively. The leached
form of Os was determined to be inactive, indicating that this system is truly heterogeneous. The
recycling study (three cycles) indicated that the catalyst can be recycled, but with a decrease in
the reaction rate (which could be due to structure defects and loss of crystallinity), and with no
significant difference in the isolated yield of the amino-alcohol.
The crystal structure of three -amino alcohols are also reported. The crystal structure of the -
amino alcohol of cyclohexene, methylcinnamte and t-butylcrotonate were needle-like triclinic,
pi, needle-like monoclinic, p21/c and cubic-like triclinic, p-1, respectively.